Extraction Technologies For Medicinal And Aromatic Plants - Unido

7 DISTILLATION TECHNOLOGY FOR ESSENTIAL OILS
still must be higher than the temperature at which the oil boils in the presence
of water on the surface of the plant material, otherwise there would
not be a temperature gradient to take the latent heat from the condensing
steam to vaporize the oil droplet. Thus, the energy from the steam in form
of heat as latent heat of vaporization converts the oil into a vapor. But, as
the boiling point of the oil is higher than that of water, the vaporization takes
place with steam on the basis of their relative vapor pressures.
It is imperative to note that a liquid always boils at the temperature
at which its vapor pressure equals the atmospheric or surrounding
pressure. For any two immiscible liquids, the total vapor pressure of
the mixture is always equal to the sum of their partial pressures. The
composition of the mixture in the vapor phase (in this case, oil and water)
is determined by the concentration of the individual components multiplied
by their respective partial pressures. For example, if a sample of
an essential oil comprised of component A (boiling point, 190° C) and
water (boiling point, 100° C) is boiled, after some time, once their vapors
reach saturation, the temperature will immediately drop to 99.5° C, which
is the temperature at which the sum of the two vapor pressures equals
760 mmHg. In other words, the oil forms an azeotropic mixture with water.
Thus, any essential oil having high boiling point can be evaporated with
steam in a ratio such that their combined vapor pressures equal the atmospheric
pressure; the essential oil can be recovered from the plant by
the wet distillation process.
7.3 Methods for Distillation
The following four techniques for the distillation of essential
oils from aromatic plants are employed:
1. Water distillation (or hydrodistillation)
2. Water and steam distillation
3. Direct steam distillation
4. Distillation with cohobation
7.3.1 Hydrodistillation
Hydrodistillation is the simplest and oldest process available
for obtaining essential oils from plants. Hydrodistillation differs from steam
distillation mainly in that the plant material is almost entirely covered with
water in the still which is placed on a furnace. An important factor to consider
in water distillation is that the water present in the tank must always be
enough to last throughout the distillation process, otherwise the plant material
may overheat and char. In this method, water is made to boil and the
essential oil is carried over to condenser with the steam which is formed.
Water-distilled oil is slightly darker in color and has much stronger still notes
than oils produced by other methods. The stills based on this principle are
116